1. Field of the Invention
The present invention relates to a method for hydrocracking a heavy fraction oil, particularly that containing at least 1.0 wt. % of asphaltene, using a hydrogen donating solvent.
2. Description of the Prior Art
The hydrogenolysis of a heavy fraction oil has recently been increasingly used. Thus, there have been proposed many method for thermocracking, catalytic cracking, hydrogenolysis, etc.
The term "cracking" used herein is intended to obtain light fraction oils including naptha, gasoline, kerosene and gas oil fractions by hydrocracking the heavy fraction oil.
The most serious and troublesome problems raised by the cracking of a heavy fraction oil are, in general, the formation of carbonaceous substances and the clogging of various parts of an apparatus for the cracking with the carbonaceous substances. Further, the serious problem caused by the catalytic cracking of the heavy fractopm oil is the decrease in catalytic activity of a catalyst used. Still further, the cracking of the heavy fraction oil raises an economic problem as to an increase in amount of hydrogen consumed. These problems are rendered more serious as the fraction oil to be cracked is heavier and the cracking proceeds farther.
One of methods for solving these problems is a method comprising the use of a hydrogen donating solvent (For example, U.S. Pat. No. 4,430,197). It is well known that compounds, such as tetralin (tetrahydronaphthalene), obtained by hydrogenating polycyclic aromatic compounds, serve as a hydrogen donor and that catalysts are not necessarily required for the hydrogenolysis of a heavy fraction oil with the use of such a hydrogen donating solvent, and these reaction proceeds under a comparatively low hydrogen pressure (For example, U.S. Pat. No. 4,294,686 and Oil & Gas Journal, Nov. 22, 1982, pp. 111-116). The above methods so known have very often been attempted to be used industrially (For example, U.S. Pat. No. 2,953,513). It is also known that such hydrogen donating substances are contained in thermocracked oils, catalytically cracked oils, hydrocracked oils and the like and function, per se, as an effective hydrogen donor (For example, U.S. Pat. No. 3,970,545).
These known methods, however, will not produce fully stable cracked products and will contain problems as to the formation of carbonaceous substances, and the like. In order to solve these problems, it is known as effective to have a suitable catalyst coexist with a hydrogen donating solvent (Japanese Pat. Appln. Laid-Open Gazette Nos. 61-62591, 61-130394, 61-136591 and U.S. Pat. No. 4,690,765.
It has been found by the present inventors that the combined use of a hydrogen donating solvent such as tetralin, and a catalyst capable of hydrogenolysis will provide the following advantages.
(1) The formation of carbonaceous substances is inhibited.
The formation of carbonaceous substances will be inhibited even by the use of a hydrogen donating solvent only. If a suitable catalyst, however, is used in combination with said solvent, the resulting cracked products will be hydrogenated and therefore stabilized whereby the formation of carbonaceous substances is greatly inhibited and troubles decrease which may otherwise be caused due to the clogging of a hydrocracking apparatus with the carbonaceous substances.
(2) A decrease in catalytic activity of the catalyst is lessened.
The most serious problem raised in the cracking of a heavy fraction oil using a suitable catalyst is a decrease in catalytic activity of the catalyst. In general, a heavy fraction oil contains asphaltene in which are contained heavy metals such as vanadium and nickel. When the heavy fraction oil is cracked, these heavy metals and carbonaceous substances will adhere to the surface of the catalyst whereby the catalyst decreases in catalytic activity. The coexistence of a hydrogen donating solvent such as tetralin in this case, will greatly lessen a decrease in catalytic activity of the catalyst.
If the catalyst used is highly capable of adhesion of heavy metals thereto at this point, the effects of the hydrogen donating solvent will be further increased.
(3) The cracking reaction may be carried out at lower pressures.
It is necessary to use a high hydrogen pressure, generally 100 atm. to 200 atm., in order to mainly prevent the catalyst from lowering in catalytic activity when cracking a heavy fraction oil in the presence of a suitable catalyst only. It is unnecessary, however, to use a high hydrogen pressure since hydrogen is supplied from a hydrogen donating solvent if the hydrogen donating solvent, such as tetralin, coexists in the system; in this case, 30 atm. to 150 atm. is sufficient as the hydrogen pressure.
(4) The amount of hydrogen consumed may be decreased.
According to the results of experiments made by the present inventors, it has been found that the cracking of a heavy fraction oil in the presence of a catalyst without a hydrogen donating solvent is different in cracking and hydrogenating reactions taking place in the cracking from the cracking of the same heavy fraction oil in the presence of the hydrogen donating solvent without the catalyst, and that both of the crackings are greatly different from each other in the amount of hydrogen consumed even in cases where the same cracking ratio or rate is obtained by each of said two cracking reactions. The combined use of the hydrogen donating solvent and the catalyst enables effective hydrogenolysis to be attained with a minimum amount of hydrogen consumed and without unnecessary hydrogenation.
It is also known that, in general, a fraction containing hydrogen donating solvents is recovered from a distillate from fractionation and the fraction so recovered is used for recirculation (Japanese Pat. Appln. Laid-Open Gazettes Nos. 61-62591 and 61-130394).
The method proposed in these Gazettes, however, raise the following problems.
(1) The starting oil is cracked, and the resulting hydrocarbon having the same boiling point as the circulating solvent is incorporated into the circulating solvent and accumulated therein whereby the concentration of tetralin in the circulating solvent decreases.
(2) In a case where a heavy fraction oil is cracked using tetralin, the tetralin and naphthalene are partly lost by carrying out distillation operation for recovering the circulating solvent. Further, the tetralin is converted to decalin, methylindane, methylnaphthalene, butylbenzene and the like during the cracking operation. The compounds produced by the conversion are discharged from the system thereby to make a partial loss of tetralin. The total amount of tetralin lost due to the distillation operation and the conversion will be less, not nullified, by suitable selection of a catalyst and cracking reaction conditions used. Accordingly, if tetralin or naphthalene (to be converted to tetralin by hydrogenation) is not produced from the starting oil during the cracking operation, the amount of tetralin or naphthalene originally existing in the system will gradually decrease. To compensate this decrease, the amount of makeup tetralin or naphthalene supplied from outside of the system will be large.